I. Field of the Invention
The present invention relates generally to cardiac rhythm management devices capable of automatically adjusting an output of the cardiac rhythm management device. More particularly, the present invention relates to a device and method for automatically monitoring and adjusting the pacing output in a cardiac pacing system so as to maintain a desired pacing output safety margin and minimize power consumption while assuring therapeutic efficacy. The device and method of the present invention does not require an actual measurement or determination of the pacing threshold value in order to maintain the desired pacing output safety margin.
II. Discussion of the Prior Art
Over the years cardiac rhythm management devices have evolved such that capture verification and pacing output thresholds may now be determined automatically. For the most part, prior art implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers, include both a sensing threshold and a pacing threshold. The success of a cardiac pacemaker in depolarizing or "capturing" the heart hinges on whether the energy of the pacing stimulus as delivered to the myocardium exceeds the hearts threshold requirements. This threshold requirement, referred to as the capture threshold or pacing output threshold, represents the amount of electrical energy required to initiate heart cell depolarization. If the energy of the pacing stimulus does not exceed the capture threshold, then depolarization will not result. If, on the other hand, the energy of the pacing stimulus exceeds the capture threshold, then depolarization results.
The sensing threshold is utilized during capture verification to determine whether a paced stimulus evokes a response having a value greater than the defined sensing threshold. The sensing threshold is necessary to avoid malsensing due to patient activity, body position, noise, or other factors. When verifying capture, the rhythm management device may have sense amplifier circuits for amplifying and filtering electrogram signals picked up by electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. Typically, the signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a source of reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as a capture or a sensed beat. The source reference potential may be referred to as a sensing threshold.
In order to maximize use of the limited power supply, it is desirable to set the lowest output energy that reliably results in capture of an electrical stimulus generated by the pulse generator, causing depolarization of the corresponding cardiac muscle. Although it is desirable to make the most efficient use of the cardiac rhythm management device's power supply, the pacing output is typically set at an energy output above the pacing threshold in order to compensate for changes in output demand. The changes in output demand may result from patient activity, body position, drugs being used, etc. To ensure the reliability of pacing, one common practice of setting the pacing output level is to determine the minimum output energy that induces a cardiac depolarization (the actual pacing output threshold), and then set the pacemaker's output at this minimum setting plus a wide error margin. The error margin is usually set at double or triple the determined effective minimum output energy. This error margin is meant to account for the changes in energy requirements that may occur between adjustments of the pacing output. The error margin may result in an unnecessary drain of the limited power supply. Also, during the determination of the actual pacing output threshold, one or more back-up paces may typically be required, thereby further draining the limited power supply.
The ability to minimize the required output energy while maintaining capture is extremely desirable in that delivering stimulation pulses having energy far in excess of the patient's capture threshold is wasteful of the pacemaker's limited power supply. Further, delivery of stimulation pulses far in excess of the patient's capture threshold may require additional complex circuitry. Further, the actual determination of the capture threshold is an added strain on the power supply. Therefore, a need exists for a rhythm management device that automatically adjusts the pacing output such that the pacing output is maintained at a level approximating the energy output of the power supply, while maintaining a desired pacing output safety margin. The present invention meets these and other needs.